Pressure measurement device

ABSTRACT

A device measures pressures in animals and humans and includes a pressure transmission catheter (PTC) filled with a pressure transmitting medium and implantable in an area in having a physiological pressure. A transducer communicates with the pressure transmitting medium to provide a pressure signal representing variations in the physiologic pressure on electrical wires. A connecting catheter carries the electrical wires to signal processing and telemetry circuitry, which transmits a telemetry signal representing the pressure signal to a receiver external to the animal or human. A housing holds the signal processing and telemetry circuitry, but the transducer is remote from the housing. The device is particularly useful in measuring venous pressure, pulmonary pressure, bladder pressure, or intracranial pressure without significant head pressure artifact and with a sufficient dynamic response. One embodiment of the PTC includes a multi-durometer stem.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of co-pending U.S. applicationNo. 09/491,233, filed Jan. 25, 2000, which is a divisional of U.S. Pat.No. 6,033,366, filed Oct. 14, 1997, the specifications of which areincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Portions of this invention were developed under Contract No. 2R44 HL55823-02, awarded by the National Institutes of Health. Therefore,the U.S. Government may have a paid-up license in portions of thisinvention and the right, in limited circumstances, to require the patentowner to license others on reasonable terms as provided for by the termsof the contract.

THE FIELD OF THE INVENTION

[0003] The present invention relates generally to implantable devices,and in particular to implantable devices for measuring variousphysiological pressures in humans or animals, such as blood pressure,intracranial pressure, bladder pressure, and pulmonary pressure.

BACKGROUND OF THE INVENTION

[0004] Measurement of physiological pressures is of interest to bothclinicians and researchers. Physiological pressure measurements obtainedfrom laboratory animals provide researchers with valuable informationregarding the safety and efficiency of pharmaceutical agents, and thetoxicity of chemicals, and leads to better understanding of humanphysiology. Physiological pressure measurements also have human clinicalvalues, such as providing diagnostic information, assessing the safetyand efficiency of drugs in clinical trials, and controlling implantablemedical devices, such as pacemakers.

[0005] Arterial blood pressure is of particular interest to the—researcher or clinician, because arterial blood pressure fluctuatesover time in response to various conditions, such as an injection ofpharmaceutical agent or chemical, or the activity level of an animalbeing observed. Arterial blood pressure fluctuations, however, oftenmake it necessary to obtain chronic, frequent measurements to identifythe effect of the injected pharmaceutical agent or chemical, or toproperly control an implantable medical device.

[0006] In addition to arterial blood pressure, other pressuremeasurements are also of interest, such as venous pressure, pulmonarypressure, intracranial pressure, bladder pressure, intrauterinepressure, gastrointestinal pressure, and other physiological pressures.For example, intrapleural or blood pressure can be used to determine therate of respiration in addition to providing general information relatedto respiratory function. Measurements of intracranial pressure fromlaboratory animals are often used to project which methods of treatmentand management are most effective in humans.

[0007] Chronic measurement of physiological pressures provides vitalinformation for clinical care of humans. Patients with high bloodpressure could benefit from an implantable device which couldchronically monitor pressure as a means of determining optimal dosagefor a drug or biofeedback therapy. Such a device could also be used as ameans of providing feedback to a closed-loop drug delivery system forcontrolling blood pressure, or to a cardiac pacemaker as a means ofoptimizing pacing control parameters.

[0008] Infants who have been identified as being at risk for suddeninfant death syndrome could also benefit. It is desirable to monitorchanges in intrapleural pressure as a reliable measurement ofrespiratory rate in these infants by means which would allow the infantto roll and move freely about its crib without being restrained by wiresextending from a vest.

[0009] Chronic monitoring of intracranial pressure is also important forinfants with hydrocephalitis and patients with head injury.Hydrocephalitis and head injuries can cause excessive pressure buildupwithin the brain, resulting in death or serious brain damage. In mostcases, corrective action can be taken if the buildup of pressure can bequickly detected.

[0010] This need to obtain accurate and ongoing physiological pressuremeasurements within various parts of animals and humans is discussed indetail in the Brockway U.S. Pat. No. 4,846,191 assigned the assignee ofthe present application, and which is herein incorporated by reference.The Brockway et al. '191 patent discloses a pressure measurement devicefor monitoring physiological pressures, such as blood pressure, invarious locations in an animal or human. The pressure measurement deviceutilizes a fluid-filled pressure transmission catheter (PTC) with a gelmembrane located at a tip of the PTC. The tip of the PTC is positionedin an area where physiological pressure is to be measured. The PTCextends from a small implantable housing that contains a transducer,signal-processing and telemetry circuitry, and a battery. Thefluid-filled PTC communicates the pressure from the area where pressureis to be measured to the transducer within the housing, which generatesan electrical pressure signal representing the communicated pressure.The signal-processing and telemetry circuitry in the housing receivesthe pressure signal generated by the transducer and provides a telemetrysignal representing the pressure signal. The signal-processing andtelemetry circuitry transmits the telemetry signal to a receiver whichis external to the animal or human.

[0011] In some applications of the pressure measurement device disclosedin the Brockway et al. '191 patent, the housing cannot be implantedwithin close proximity to the area where pressure is to be measured dueto physical limitations and practical considerations of surgicalprocedures. When the housing is not within close proximity to the areawhere pressure is to be measured, the length of the catheter that isrequired may be too long to assure that errors, resulting from decreaseddynamic response or changes in posture, be within acceptable limits forthe given application. For example, if the vertical distance from thePTC tip to the transducer changes due to posture, an error in thepressure measurement occurs. Every one centimeter change in verticaldistance creates approximately one millimeter Hg error in the pressuremeasurement for one preferred low-viscosity fluid used in the catheter.This pressure measurement error is known as “head pressure artifact” andis very significant in certain applications. Furthermore, as the lengthof the PTC increases, the dynamic response of the pressure measurementdevice is reduced. In certain applications, the required length of thePTC is so long that a sufficient dynamic response cannot be obtained.

[0012] In addition, the Brockway et al. '191 patent does not disclose apre-compensated, disposable, and easily replaceable transducer. Rather,since the transducer is inside the housing, if the transducer disclosedin the Brockway et al. '191 patent fails, the entire pressure-sensingdevice must be returned to the manufacturer for replacement to ensureproper compensation, mounting of the transducer, and sealing of theimplant body.

[0013] For reasons stated above and for other reasons presented ingreater detail in the Description of the Preferred Embodiments sectionof the present specification, there is a need for a pressure measurementdevice that is capable of measuring pressures in more animal and humanapplications, with better dynamic response, and with more accuratepressure measurements than currently possible with present pressuremeasurement devices. In addition, it is desired that the transducer andcatheter of the pressure measurement device be more easily replaceablethan currently possible with present pressure measurement devices.

SUMMARY OF THE INVENTION

[0014] The present invention provides a pressure measurement devicewhich measures physiological pressures in animals and humans. Thepressure measurement device includes a pressure transmission catheterfilled with a pressure transmitting medium and implantable in an areahaving a physiological pressure. A transducer is in communication withthe pressure transmitting medium to provide a pressure signalrepresenting variations in the physiologic pressure on electrical wires.A connecting catheter carries the electrical wires to signal processingand telemetry circuitry, which receives the pressure signal and providesa telemetry signal representing the pressure signal. A housing holds thesignal processing and telemetry circuitry. The transducer is remote fromthe housing.

[0015] The pressure transmission catheter preferably has a length shortenough to avoid significant head pressure artifact and to providesufficient dynamic response, but long enough to accommodate surgicallimitations and tolerance concerns. For example, depending on theparticular application of the pressure measurement device, the pressuretransmission catheter typically has a length somewhere in the range fromapproximately five millimeters to approximately four centimeters. Inmost applications, the pressure transmitting medium comprises a gel anda liquid. Nevertheless, because the present invention permits thepressure transmission catheter to be significantly shorter thanpreviously possible, in certain applications, the pressure transmittingmedium includes only a gel. In one embodiment, the transducer isintegral with the pressure measurement catheter to form atransducer-tipped catheter.

[0016] The pressure measurement device according to the presentinvention can be employed to accurately measure low pressure where headpressure artifact can constitute a significant percentage of thepressure being measured. These pressures include: venous pressure;pulmonary pressure; intracranial pressure; bladder pressure; and otherpressures. The pressure measurement device measures these pressureswithout significant head pressure artifact and with a sufficient dynamicresponse.

[0017] The transducer is preferably pre-temperature compensated anddisposable. In this way, the transducer, which is external to thehousing, can be easily replaced without replacing the entire pressuremeasurement device. In many applications of the pressure measurementdevice, the housing is implantable remote from the area having thephysiological pressure.

[0018] In one form of the invention, the pressure transmitting catheterincludes a lumen filled with the pressure transmitting medium. An innerlayer material surrounds the lumen and an outer layer material surroundsthe inner layer material. The outer layer material is of a differenthardness than the inner layer material. In a preferred embodiment, theinner layer material is harder then the outer layer material.Preferably, the harder layer material essentially determines thefrequency response of the pressure transmitting catheter so thatcompared to a catheter fabricated of only softer material, the catheterof the present invention provides improved frequency response.Preferably, the softer layer material makes the pressure transmittingcatheter more flexible and kink resistant compared to a catheterfabricated of only harder material. A transition between the inner layermaterial and the outer layer material can be a sharp transition or agradient transition. In one embodiment, the inner layer materialcomprises 72 D urethane and the outer layer material comprises 80 Aurethane.

[0019] The pressure measurement device according to the presentinvention achieves more accurate measurement of physiological pressureand can be employed in many new applications for pressure measurement inanimals and humans. The pressure measurement device according to thepresent invention obtains high-fidelity measurements with negligiblehead pressure error in applications where the distance from the distaltip of the pressure transmission catheter to the transmitter is suchthat significant head pressure errors could occur with conventionaldevices.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a diagram of a pressure measurement device according tothe present invention.

[0021] FIGS. 2A-D are slightly more detailed diagrams of variousembodiments of a portion of a pressure transmission catheter (PTC)employed in the pressure measurement device of FIG. 1.

[0022]FIG. 3 is a slightly more detailed diagram of one embodiment of atransducer and the transducer's connections to the PTC and a connectingcatheter employed in the pressure measurement device of FIG. 1.

[0023]FIG. 4 is a diagram of an alternative embodiment of pressuremeasurement device according to the present invention.

[0024]FIG. 5 is a diagram illustrating a cross-section of amulti-durometer catheter according to the present invention.

[0025]FIG. 6 is a diagram illustrating the application of the pressuremeasurement device of FIG. 1 to measure venous pressure.

[0026]FIG. 7 is a diagram illustrating the application of the pressuremeasurement device of FIG. 1 to monitor pulmonary pressure.

[0027]FIG. 8 is a diagram illustrating the application of the pressuremeasurement device of FIG. 4 to monitor intracranial pressure.

[0028]FIG. 9 is a diagram of an alternative pressure measurement device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] In the following detailed description of the preferredembodiments, reference is made to the accompanying drawings which form apart hereof, and in which is shown by way of illustration specificembodiments in which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural orlogical changes may be made without departing from the scope of thepresent invention. The following detailed description, therefore, is notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims.

[0030] A pressure measurement device according to the present inventionis illustrated generally at 20 in FIG. 1. Pressure measurement device 20is a miniature implantable device capable of measuring internalphysiological body pressure in humans or animals. The fundamentalprinciples by which pressure measurement device 20 measures pressure aredescribed in detail in the Brockway et al. '191 patent, which wasincorporated by reference in the Background of the Invention section ofthe present specification. Therefore, for clarity, many of the featuresof pressure measurement device 20 which are similar to the pressuremeasurement device described in the Brockway et al. '191 patent are notdiscussed herein. However, the features which differ from the pressuremeasurement device described in the Brockway et al. '191 patent arediscussed in detail below.

[0031] Pressure measurement device 20 includes a pressure transmissioncatheter (PTC) 22 having a distal lumen tip 24 which is positionedwithin the body of a human or animal at the site where pressure is to bemeasured. One embodiment of PTC 22 is flexible, while another embodimentof PTC 22 is rigid, and the particular embodiment of PTC 22 selecteddepends on the given application of pressure measurement device 20. PTC22 is filled with a pressure-transmitting medium 26 which communicatesthe pressure at the distal tip 24 of PTC 22 to a proximal lumen end 28of PTC 22. Thus, a portion of pressure-transmitting medium 26 at distaltip 24 interfaces with the substance in the body area where pressure isto be measured, such as with blood in an artery.

[0032] A transducer 30 is in communication with pressure-transmittingmedium 26 at the proximal end 28 of PTC 22. Transducer 30 is containedin a transducer housing 32. Transducer 30 responds to variations in thepressure-transmitting medium at proximal end 28 to provide an electricalpressure signal representing variations in the physiological pressure atdistal tip 24 on electrical lead wires 34. Electrical leads wires 34 arecarried within a connecting catheter 36. By separating PTC 22 fromconnecting catheter 36, the length of connecting catheter 36, indicatedby arrows 48, can be independently determined from the length of PTC 22,indicated by arrows 50. The length of connecting catheter 36, indicatedby arrows 48, varies from zero to very long, depending on theapplication of pressure measurement device 20.

[0033] Electrical lead wires 34 are coupled to an electronics module 38of a transmitter 40. Electronics module 38 is powered by a battery 42.Battery 42 and electronics module 38 are contained within a transmitterhousing 44. The electronics module 38 includes signal-processing andtelemetry circuitry and a transmitting antenna for generating andtransmitting a telemetry signal representing the pressure signal fromtransducer 30 to an external receiver (not shown) disposed outside ofthe human or animal. The electrical pressure signal produced bytransducer 30 is amplified and filtered with the signal-processingcircuitry in electronics module 38 and is then modulated onto aradio-frequency carrier by the telemetry circuitry in electronics module38 for transmission to the external receiver. A suitable telemetrysystem is described in the Brockway et al. '191 patent and the patentapplication entitled “Respiration Monitoring System Based on SensedBlood Pressure Variations,” Ser. No. 08/535,656, filed Sep. 28, 1995,assigned to the assignee of the present application, and which is hereinincorporated by reference.

[0034] In one embodiment of pressure measurement device 20, connectingcatheter 36 is coupled to transmitter 40 with a water-tight ornon-water-tight connector 46. In this embodiment, the electrical leadwires 34 contained connecting catheter 36 interface with electronicsmodule 38 via connector 46. Water-tight or non-water-tight connector 46permits an assembly 47 including connecting catheter 36, transducer 30,and PTC 22 to be manufactured and sold separately from transmitter 40.Connector 46 also provides more flexibility for manufacturing assembly47 and transmitter 40 and additional flexibility for customers inselecting lengths of connecting catheter 36 and PTC 22 independent fromtransmitter 40.

[0035] One embodiment of PTC 22 is illustrated in more detail in FIG.2A. In this embodiment of PTC 22, a viscous gel membrane 52 is disposedat the distal tip 24 of PTC 22. A thin-walled section 54 defines an opencavity 56. A stem 55 of PTC 22 runs from the thin-walled section 54 toproximal end 28 of PTC 22, as illustrated in FIG. 1. As illustrated inFIG. 2A, the gel membrane 52 is contained in a distal portion of opencavity 56. Open cavity 56 is connected to a lumen 58 of PTC 22. Theportion of open cavity 56 not filled with viscous gel 52 and lumen 58are filled with a low-viscosity fluid 60. In this way, physiologicalpressure is transmitted from distal tip 24 of PTC 22 through the wallsof the PTC and via viscous gel 52 contained within thin-walled section54 to the low-viscosity fluid 60 which communicates pressure directly totransducer 30 at the proximal end 28 of lumen 58. The low frequencycomponents of the physiological pressure are essentially transmitted viaviscous gel 52 while the high frequency components of the physiologicalpressure are essentially transmitted through the walls of the PTC.

[0036] In one embodiment, PTC 22 is fabricated of a urethane material orother suitable biocompatible material. Viscous gel membrane 52 is abiocompatible and blood-compatible gel or other gel-like material thatprovides a direct interface with the tissue or fluid from which pressureis to be measured, such as blood in an artery. Viscous gel 52 provides ameans of retaining fluid within lumen 58 and is of a viscosity muchhigher than that of low-viscosity fluid 60. Viscous gel 52 can becomprised of any material which is capable of flowing or moving withinPTC 22 as does a viscous fluid or a plug that can slide or deform easilyand contains intramolecular forces which make it very unlikely that anyportion of this material will dissolve, break apart, slough off, or washaway when measuring physiological pressure within a human or animal.Viscous gel 52 must be viscous enough not to wash out of PTC 22, butalso must be low enough in viscosity that it can “flow” withoutsignificant pressure differential. In one embodiment of the invention,viscous gel 52 is a silicone gel which contains cross-linked molecularentities.

[0037] Low-viscosity fluid 60 preferably has a minimal biologicalactivity (in case of failure of a seal), has a low thermal coefficientof expansion, is insoluble in gel 52, has a low specific gravity, has anegligible rate of migration through the walls of PTC 22, and has a lowviscosity at body temperature. In one embodiment, low-viscosity fluid 60is an inert perfluorocarbon.

[0038] In other embodiments of the pressure measurement device accordingto the present invention, PTC 22, which can be rigid or flexible, isvery short, and can be as short as approximately 2 mm long. One suchembodiment is illustrated in FIG. 2B. In the embodiment illustrated inFIG. 2B, since the length of PTC 22 is very short, PTC 22 is typicallyfilled entirely with viscous gel 52 (i.e, the low-viscosity fluid 60 isnot used), but still provides a sufficient dynamic response.

[0039] The thin-walled section 54 reduces movement of viscous gel 52during events that change either the volume of low-viscosity fluid 60 orthe internal volume of lumen 58 of PTC 22, such as occurs during thermalexpansion and contraction, bending, and hydration of the cathetermaterial of PTC 22. Reducing the degree of displacement of gel 52 duringbending of PTC 22 has the effect of reducing measurement artifact thatcan occur during normal movement of the human or animal into whichpressure measurement device 20 is implanted. Reducing the degree ofdisplacement of gel 52 during bending of PTC 22 reduces the amount ofdead space within PTC 22 and beyond gel 52, and therefore, contributesto improved patency in blood. Thin-walled section 54 also improves thefrequency response of PTC 22 by providing a means by which to transferhigh-frequency components of the pressure signal into lumen 58 throughthe compliant thin walls of the tip.

[0040] Two additional embodiments of a PTC 22 are illustrated in FIGS.2C and 2D. In these embodiments, PTC 22 does not include an open cavity56 defined by a thin-walled section 54, but instead, the small diameterportion of lumen 58 runs all the way to the distal tip 24. Thisembodiment can be used in certain applications where the aboveadvantages of having such a thin-walled section are not as significantto obtaining satisfactory pressure measurements. In the embodiment ofPTC 22 illustrated in FIG. 2C, a viscous gel membrane 52 is disposed atthe distal tip 24 of PTC 22 with the remainder of lumen 58 being filledwith a low-viscosity fluid 60. In the embodiment of PTC 22 illustratedin FIG. 2D, the PTC is filled entirely with viscous gel 52 (i.e., thelow-viscosity fluid 60 is not used).

[0041] A more detailed diagram of one embodiment of transducer 30 andthe coupling of transducer 30 to PTC 22 and to connecting catheter 36 isillustrated in FIG. 3. As illustrated, PTC 22 is attached to transducer30 via a nipple 62. In one embodiment of the invention, transducerhousing 32 comprises a hermetic titanium housing. Transducer 30 iscontained within a sealed chamber 64. Sealed chamber 64 protectstransducer 30 from body fluids. Electrical connections from transducer30 are coupled to a circuit board 68. Circuit board 68 includescircuitry 67 employed for temperature-compensating transducer 30. In analternate embodiment, temperature of transducer 30 is measured by asensor 69 on circuit board 68 and a remote computing device (not shown)employs these temperature measurements to temperature-compensatetransducer 30. Electrical connections from circuit board 68 pass out ofsealed chamber 64 via glass-metal seals 70 to thereby connect toelectrical lead wires 34 contained within connecting catheter 36.

[0042] Although transducer 30 is typically smaller than the transduceremployed in the pressure measurement device described in the Brockway etal. '191 patent to permit transducer 30 to be disposed remote from thetransmitter housing 44, the general operation and construction of asuitable transducer 30 is described in detail in the Brockway '191patent.

[0043] An alternative embodiment pressure measurement device 120 ispartially illustrated in FIG. 4. Pressure measurement device 120 issimilar to pressure measurement device 20 illustrated in FIG. 1.However, PTC 22 couples to transducer housing 32 at a right angle inpressure measurement device 120. This is made possible with a nipple 162which is L-shaped to receive the proximal end 28 of lumen 58 of PTC 22to couple the low-viscosity fluid 60 to transducer 30. This right-angleembodiment is only one of many examples of the great flexibilityprovided by having PTC 22 separated from connecting catheter 36 andhaving transducer 30 being remote from transmitter housing 44.

[0044] In the preferred embodiment of the pressure measurement devicedescribed in the Brockway et al. '191 patent, a transmitter housinghouses an electronics module, a battery to power the electronics module,and a transducer. For some applications, locating the transducer withinthe transmitter housing creates certain disadvantages to sensingaccurate pressure. For example, for one preferred low-viscosity fluid60, a head pressure error is created approximately equal to one mm Hgfor every one cm of vertical distance between the tip of the PTC and thetransducer. This head pressure error can be very significant relative tothe pressures being measured in some applications. Another disadvantageis that the dynamic response of the PTC is inversely proportional to itslength. Therefore, when the required length of the PTC becomes too longin some applications, the dynamic response is reduced to a level whichis not sufficient to reproduce a high-fidelity waveform. Still anotherdisadvantage of the preferred embodiment described in the Brockway etal. '191 patent is that for some combinations of material employed inthe PTC, a greater volume of low-viscosity fluid contained in the lumenof the PTC results in a greater degree of thermal expansion andcontraction, and greater degree of movement of the gel membrane. Ifmovement of the gel membrane is too great, a void can develop within thetip of the PTC resulting in dead space thrombosis. Consequently, keepingthe PTC short reduces the volume of low-viscosity fluid contained in thelumen of the PTC which contributes to improved patency in blood in someapplications.

[0045] The pressure measurement device according to the presentinvention, such as pressure measurement device 20, overcomes all of theabove disadvantages by providing a means of shortening the requiredlength of PTC 22 in many applications. This length of PTC 22 is from thedistal tip 24 to the proximal end 28 and is indicated by arrows 50. Thereduced length 50 can greatly reduce head pressure error and improve thedynamic response to a degree which is acceptable to the researcher andclinician using the pressure measurement device.

[0046] A much shorter PTC is achievable because pressure measurementdevice 20 disposes transducer 30 remote from transmitter housing 44. Inaddition, pressure measurement device 20 employs a connecting catheter36 which is separated from PTC 22 to carry the electrical leads 34 whichcouple the pressure signal from transducer 30 to electronics module 38of transmitter 40. This permits the length of connecting catheter 36 andPTC 22 to be independently determined.

[0047] Head pressure error is significantly reduced as the distal tip ofPTC 22 and transducer 30 can be brought much closer together in manyapplications. Since the dynamic response is inversely proportional tothe length 50 of PTC 22, a shorter PTC 22 increases the dynamic responseto permit reproduction of high-fidelity waveforms in many applicationswhere previous pressure measurement devices, having too long of PTC,cannot reproduce high-fidelity waveforms. In addition, the volume oflow-viscosity fluid 60 contained in lumen 58 of PTC 22 is reduced withthe significantly shorter PTC 22 in some applications. The reducedvolume results in significantly less thermal expansion and contractionand less degree of movement of viscous gel membrane 52. With lessmovement of gel membrane 52, voids are avoided at distal tip 24 of PTC22 to prevent dead space thrombosis. The shorter PTC 22, therefore,reduces the volume of low-viscosity fluid 60 to thereby improve patencyin blood. Moreover, as discussed above with reference to FIG. 2B,because PTC 22 can be very short in many applications, PTC 22 isalternatively completely filled with viscous gel 52 in certainapplications and still performs acceptably. Since transducer housing 32is much smaller than transmitter housing 44, it is possible to locatetransducer 30 much closer to the pressure source with much lessphysiologic impact and better convenience from a surgical perspective.

[0048] The present invention also eliminates certain logistical problemsfor pressure measurement devices in the animal market. Presently, if acatheter is damaged or if a customer accidentally applies too muchpressure and bursts the transducer, it is necessary to send the entirepressure measurement device back to the factory. With transducer 30located remote from transmitter housing 44 and connected electrically totransmitter 40 via water-tight or non-water-tight connector 46 and theelectrical lead wires 34 in connecting catheter 36, transducer 30 caneasily be detached. In this way, pre-compensated transducers 30 can besold to the customer as a disposable product and be attached by thecustomer without the need to return transmitter 40 for repair.

Multi-durometer Catheter

[0049] As discussed above, the present invention permits a significantreduction in the required length of PTC in certain applications.Nevertheless, there are many characteristics of the fluid/gel-filled PTCwhich are critical in order to assure that the pressure communicated totransducer 30 is an accurate representation of the physiologicalpressure present at the distal tip 24 of PTC 22. In particular, if PTC22 is not capable of transmitting high-frequency components of thephysiological pressure at distal tip 24 to transducer 30, pressureinformation which is not transmitted causes an inaccurate representationof the physiological pressure to be produced by transducer 30.

[0050] Physical characteristics which affect the ability of PTC 22 toaccurately transmit the physiological pressure include: the viscosity ofthe fluid within the PTC; the surface area of thin-walled section 54that is exposed to low-viscosity fluid 60; the compliance of the wallsof the PTC; the inner diameter of the PTC; and the length of the PTC. Asto the viscosity of the fluid, viscosity of the fluid is dependent uponavailable materials and is to a large extent out of the control of thedesigner. As discussed above, the present application describes apressure measurement device 20 having transducer 30 disposed between PTC22 and transmitter 40 where connecting catheter 36 carries wires tocouple the pressure signal from the transducer to the transmitter. Thissignificantly reduces the required length of the PTC in certainapplications. The inner diameter of the PTC and the compliance of thewalls of the PTC are two factors that greatly affect the fidelity of themeasured pressure. Once a critical inner diameter is reached, furtherreduction results in a rapid drop in frequency response of the PTC. Thefrequency response of the PTC improves as the compliance of the PTCwalls is lowered (i.e., the stiffness of the walls is increased). Thecompliance of the walls is a function of the materials employed tofabricate the PTC, the construction of the PTC, and the thickness of thePTC walls.

[0051] In current commercially available pressure measurement deviceshaving fluid-filled PTCs to refer pressure from a point of interest to atransducer, the outer diameter of the PTC is sufficiently large topermit the inner diameter to be sufficiently large and the PTC walls tobe sufficiently thick to provide a sufficient frequency response.However, when a physiological pressure is measured in very smallvessels, such as those in mice or in human coronary arteries, therequired PTC outer diameter typically ranges from approximately 0.014to0.022 inches. In such a PTC, the inner diameter of PTC stem 55 must bevery small (e.g., less than approximately 0.008 inches) and the walls ofthe PTC stem must be very thin (e.g., less than approximately 0.005inches). The very small inner diameter of PTC stem 55 and the thin wallsof the PTC stem result in reduced frequency response when a flexiblethermoplastic is used to fabricate the PTC.

[0052] One approach to solve the frequency response problem caused bythin PTC stem walls and a very small diameter PTC stem is to fabricatePTC stem 55 of a hard material, such as 75 Shore D utethane. However, aPTC stem fabricated of this hard material is too stiff to be handledeasily during surgery and kinks too easily. Another approach to improvethe frequency response of a PTC stem having a very small inner diameterand thin walls is to reduce the compliance of the walls by winding awire in a helix around the PTC stem. Nevertheless, this wire-aroundapproach is expensive to manufacture and is very difficult to do in areliable manner when the wall thickness of the PTC is less than 0.004inches.

[0053] A cross-section of a multi-durometer catheter according to thepresent invention is illustrated generally at 200 in FIG. 5.Multi-durometer catheter 200 forms the stem portion of a fluid-filledcatheter (PTC), such as the stem 55 of PTC 22 described above. This stemportion transfers pressure from the tip of the PTC to the transducer.

[0054] Multi-durometer catheter 200 includes a lumen 202. Lumen 202 issurrounded by an inner layer of harder material 204. The inner layer ofharder material 204 is surrounded by an outer layer of softer (morecompliant) material 206. The transition between the inner layer ofharder material 204 and the outer layer of softer material 206 isdefined by an interface 208. The interface 208 can be a gradientinterface to gradually transition from the inner layer to the outerlayer or can alternatively be a sharp transition interface between theinner layer and the outer layer.

[0055] Multi-durometer catheter 200 has an outer diameter indicated byarrows 210 and defined by an outer surface 212. The outer diameterindicated by arrows 210 is typically in the range from approximately0.014 to 0.022 inches. A wall 214 of catheter 200 is formed by the innerlayer of harder material 204 and the outer layer of softer material 206.With the outer catheter diameter being in the range of approximately0.014 to 0.022 inches, the inner diameter of lumen 202, as indicated byarrows 216, is typically less than approximately 0.008 inches and thethickness of wall 214, indicated by arrows 218, is typically less thanapproximately 0.005 inches. In one embodiment of a multi-durometercatheter 200 having such dimensions, the thickness of the inner layer ofharder material 204, indicated by arrows 220, is less than approximately0.002 inches. Thus, in this embodiment the thickness of the outer layerof softer material 206, indicated by arrows 222, is approximately lessthan 0.003 inches.

[0056] The type of satisfactory harder material 204 and softer material206 which can be employed to fabricate catheter 200 vary depending theparticular implementation and application of the PTC. In one embodiment,the inner layer of harder material 204 comprises 72 D urethane with theouter layer of softer material 206 comprising 80 A utethane. With thisembodiment of a 5 cm long multidurometer catheter 200, having the abovedimensions, for a hydrated catheter at 37° C., the drop in frequencyresponse is only approximately 1 dB at 100 Hz. In some applications,there can possibly be delamination between the two durometers ofurethane represented by inner layer of harder material 204 and outerlayer of softer material 206. This delamination problem is solved bycreating a chemical bond between inner layer of harder material 204 andouter layer of softer material 206 during a real co-extrusion.

[0057] Multi-durometer catheter 200 is inexpensive and easy to fabricateusing a true co-extrusion technique or by extruding the outer layer overthe inner layer by making a second pass through the extruder. The trueco-extrusion technique typically obtains a gradient transition betweenthe inner layer and outer layer and the two pass extrusion techniquetypically obtains a sharp transition between the inner layer and outerlayer.

[0058] The multi-durometer catheter 200 according to the presentinvention provides a means of producing a good frequency response in athin-walled, very small-diameter catheter while maintaining excellentflexibility. The compliance of catheter 200 is primarily derived by theproperties of the harder material 204 to produce sufficient frequencyresponse. Nevertheless, the layer of softer material 206 makes catheter200 kink resistant and flexible. Thus, catheter 200 is easily handledand kink resistant during surgery. In applications where a flexiblecatheter 200 is not required, a rigid multi-durometer catheter 200 stillprovides better frequency response than conventional rigidsingle-durometer catheters.

[0059] In one embodiment of multi-durometer catheter 200, inner layer ofharder material 204 includes a radiopaque material. For example, in oneembodiment, the inner layer of harder material 204 includes anapproximately 20-30% barium filled 72 D urethane. One problem withintroducing a radiopaque material into a single-durometer catheter isthat radiopaque materials are often thrombogenic. This problem is solvedwith this embodiment of multi-durometer catheter 200, because outerlayer of softer material 206 comprises only non-thrombogenic material.In this way, this embodiment of multi-durometer 200 is radiopaque andnon-thrombogenic. Alternatively, a softer inner layer having aradiopaque material could be disposed between the inner layer of hardermaterial 204 and the outer layer of softer material 206.

[0060] One alternative embodiment of a multi-durometer catheteraccording to the present invention comprises an outer layer of hardermaterial and an inner layer of softer material. In this alternativeembodiment, the compliance of the multi-durometer catheter is stillprimarily derived by the properties of the harder material to producesufficient frequency response and the layer of softer material makes thecatheter kink resistant.

[0061] Example Applications of the Pressure Measurement Device Accordingto the Present Invention

[0062] The pressure measurement device can be advantageously used toobtain pressures in animals and humans for all of the example usesdisclosed in the Brockway et al. '191 patent, such as: for measuringblood pressure in an artery of an animal; for measuring intrapleuralpressure in animals; and for measuring intracranial pressure in animals.However, the following three example applications of the pressuremeasurement device of the present invention illustrate threeparticularly useful applications, which take advantage of the featuresof the present invention, such as having a reduced length PTC 22, havinga separate PTC 22 and connecting catheter 36, and having the transducer30 being remote from transmitter housing 44.

[0063] Application of Pressure Measurement Device for Measuring VenousPressure

[0064]FIG. 6 illustrates an embodiment of the pressure measurementdevice 20 according to the present invention for measuring venouspressure, such as venous pressure in a laboratory rat. Venous pressureis relatively low and head pressure error can thus be significant andhighly undesirable. In this embodiment, the length of PTC 22 (indicatedby arrows 50 in FIG. 1) is typically approximately 4 centimeters longand the length of connecting catheter 36 (indicated by arrows 48 inFIG. 1) is typically approximately 25 centimeters long. In thisapplication, PTC 22 typically comprises an approximately 2-3 mm long gelplug 52 at the distal tip 24, with the remainder of PTC 22 filled withlow-viscosity fluid 60.

[0065] To surgically implant a pressure measurement device according tothe present invention in a vein of a rat or other laboratory animal, avein 80 is exposed, such as an abdominal vein. PTC 22 is inserted intovein 80 to sense pressure of blood 81 and is secured at a point wherePTC 22 exits vein 80 using medical grade tissue adhesive or apurse-string suture. As illustrated in FIG. 6, about one-half of theapproximately 4 centimeter long PTC 22 is inserted into vein 80.Transducer housing 32 containing transducer 30 is disposed outside ofvein 80 and is secured to tissues at a point immediately adjacent tovein 80 and as near to distal tip 24 of PTC 22 as possible. Transmitter40 is secured to a muscle or within a subcutaneous pocket at a sitewhich is convenient to the surgeon performing the procedure. Forexample, when PTC 22 is inserted into the abdominal vein of a laboratoryrat, transducer housing 32 is typically sutured to the muscle next tothe abdominal vein. In this application, transmitter housing 44 istypically sutured to a ventral abdominal muscle at the incision made toaccess the abdomen.

[0066] Application of Pressure Measurement Device for MonitoringPulmonary Pressure

[0067]FIG. 7 illustrates an embodiment of the pressure measurementdevice 20 according to the present invention employed to monitorpulmonary pressure in a human. In this embodiment, PTC 22 is typicallyapproximately 1-2 cm long and connecting catheter 36 is typicallyapproximately 50 cm long. In this application, connecting catheter 36must be quite long (e.g., approximately 50 cm). Since the monitoredpulmonary pressure is low, head pressure artifact is a problem overcomeby using the short PTC 22 (e.g., approximately 1-2 cm). In thisapplication PTC 22 typically contains both viscous gel membrane 52 andlow-viscosity fluid 60. Nevertheless, in this application, PTC 22 can becompletely filled with viscous gel 52, such as illustrated in FIG. 2B,and still perform acceptably, because of the very short length of PTC22. In addition, depending on the thermal characteristics of PTC 22,transducer 30, and low-viscosity fluid 60, this application optionallyemploys a PTC 22 without a larger diameter thin-walled section 54, suchas illustrated in FIG. 2C.

[0068] To surgically implant the pressure measurement device 20according to the present invention in this application, PTC 22,transducer housing 32, and connecting catheter 36 are inserted intosubclavian vein 82, passed into right ventricle 84 of heart 86, andguided out of heart 86 through pulmonary semilunar valve 88 intopulmonary artery 90. Following the above procedure to position PTC 22and connecting catheter 36, transmitter housing 44 is disposed in asubcutaneous pocket 92 near the site of entry to subclavian vein 82. Asillustrated in FIG. 7, in this application of the pressure measurementdevice 20 according to the present invention, the complete length of PTC22 and a large portion of connecting catheter 36 reside within thecirculatory system.

[0069] Application of Pressure Measurement Device for MonitoringIntracranial Pressure

[0070] An embodiment of the pressure measurement device 120 according tothe present invention for monitoring intracranial pressure isillustrated in FIG. 8. Intracranial pressures are relatively low andthus head pressure errors can be significant. In this application, it istypically undesirable for transmitter housing 44 to be placed beneaththe scalp because it may be uncomfortable due to its size. Therefore,transmitter housing 44 containing transmitter 40 is typically placedsubcutaneously, as indicated at 94, in a convenient location on the neckof the patient or on the upper ventral thorax, based on surgeonspreference. PTC 22 is typically approximately 1.5 cm long and preferablyexits transducer housing 32 at an approximately 90° angle. Thus, theright angle pressure measurement device 120 illustrated in FIG. 4 ispreferable used in this application. The right angle provides a moreconvenient surgical placement. In this application, connecting catheter36 is typically approximately 70 cm long. In this application, PTC 22typically includes viscous gel membrane 52 at distal tip 24 with theremainder of PTC 22 filled with low-viscoaity fluid 60.

[0071] Surgical implantation in this application involves making asubcutaneous pocket 96 at the location 94 where transmitter 40 is to beplaced. PTC 22 and connecting catheter 36 are directed under the skinfrom location 94 to a location 97 within cranium 98 where pressure is tobe monitored. Following exposure of cranium 98 at the location 97 wherepressure is to be monitored, a hole is drilled through cranium 98. PTC22 is then directed through the hole into a subarachnoid space 100. Toprevent transducer housing 32 from extending above the normal plane ofthe scalp, a shallow cavity 102 is formed in cranium 98. Transducerhousing 32 is placed in shallow cavity 102. In this embodiment,transducer housing 32 is constructed with a flat profile to inhibitmigration under the skin and to improve tolerance by the patient.

[0072] Alternative Pressure Measurement Device

[0073] An alternative pressure measurement device is illustratedgenerally at 320 in FIG. 9. Pressure measurement device 320 includes apressure transmission catheter (PTC) 322 having a distal lumen tip 324which is positioned within the body of a human or animal at the sitewhere pressure is to be measured. One embodiment of PTC 322 is flexible,while another embodiment of PTC 322 is rigid, and the particularembodiment of PTC 322 selected depends on the given application ofpressure measurement device 320. PTC 322 is filled entirely with apressure-transmitting gel 352 which communicates the pressure at thedistal tip 324 of PTC 322 to a proximal lumen end 328 of PTC 322. Thus,a portion of pressure-transmitting gel 352 at distal tip 324 interfaceswith the substance in the body area where pressure is to be measured,such as with blood in an artery.

[0074] A transducer 330 is in communication with pressure-transmittinggel 352 at the proximal end 328 of PTC 322. As illustrated, PTC 322 isattached to transducer 330 via a nipple 362. Transducer 330 is containedin a transducer housing 332. Transducer 330 responds to variations inthe pressure-transmitting gel at proximal end 328 to provide anelectrical pressure signal representing variations in the physiologicalpressure at distal tip 324 on electrical lead wires, which are coupledto an electronics module 338 of a transmitter 340. Electronics module338 is powered by a battery 342. Battery 342, electronics module 338,and transducer housing 332 are contained within a transmitter housing344. The electronics module 338 includes signal-processing and telemetrycircuitry and a transmitting antenna for generating and transmitting atelemetry signal representing the pressure signal from transducer 330 toan external receiver (not shown) disposed outside of the human oranimal. The electrical pressure signal produced by transducer 330 isamplified and filtered with the signal-processing circuitry inelectronics module 338 and is then modulated onto a radio-frequencycarrier by the telemetry circuitry in electronics module 338 fortransmission to the external receiver.

[0075] In alternative pressure measurement device 320, the transducer330 is contained within transmitter housing 344, and PTC 322 is filledentirely with pressure-transmitting gel 352 (i.e., a low viscosity fluidis not used). This embodiment can be employed in applications where PTC322 is very short and the transmitter housing 344 is sufficiently smallto enable it to be located at the pressure source.

[0076] Pressure transmitting gel 352 is a biocompatible andblood-compatible gel or other gel-like material that provides a directinterface with the tissue or fluid from which pressure is to bemeasured, such as blood in an artery. Gel 352 provides a means ofretaining fluid within PTC 322 and can be comprised of any materialwhich is capable of flowing as does a viscous fluid and containsintramolecular forces which make it very unlikely that any portion ofthis material will dissolve, break apart, slough off, or wash away whenmeasuring physiological pressure within a human or animal. Gel 352 mustbe viscous enough not to wash out of PTC 322, but also must be lowenough in viscosity that it can “flow” without significant pressuredifferential. In one embodiment of the invention, pressure transmittinggel 352 is a silicone gel which contains cross-linked molecularentities.

Conclusion

[0077] The pressure measurement device according to the presentinvention can be employed to sense numerous internal body pressures inhumans and animals including pulmonary pressure, venous pressure, leftventricle pressure, intracranial pressure, bladder pressure, and otherphysiological pressures. Pressure information sensed with the pressuremeasurement device according to the present invention is available fordiagnostic purposes, research, or feedback for closed-loop control ofinfusion pumps capable of administering pharmaceutical agents.

[0078] The pressure measurement device according to the presentinvention, such as pressure measurement device 20, overcomes theabove-discussed disadvantages of the previous pressure measurementdevices, offers significant new opportunities for more accuratemeasurement of pressure, and opens new applications for pressuremeasurement in animals and humans. The pressure measurement deviceaccording to the present invention obtains high-fidelity measurementswith negligible head pressure error in applications where the distancefrom the distal tip of the PTC to the transmitter is large.

[0079] Although specific embodiments have been illustrated and describedherein for purposes of description of the preferred embodiment, it willbe appreciated by those of ordinary skill in the art that a wide varietyof alternate and/or equivalent implementations calculated to achieve thesame purposes may be substituted for the specific embodiments shown anddescribed without departing from the scope of the present invention.Those with skill in the mechanical, electromechanical, electrical, andcomputer arts will readily appreciate that the present invention may beimplemented in a very wide variety of embodiments. This application isintended to cover any adaptations or variations of the preferredembodiments discussed herein. Therefore, it is manifestly intended thatthis invention be limited only by the claims and the equivalentsthereof.

What is claimed is:
 1. An apparatus for measuring physiological pressure comprising: a pressure transmission catheter filled with a pressure transmitting medium and implantable in an area having a physiological pressure; a connecting catheter carrying electrical wires; a transducer coupled to the electrical wires and in communication with the pressure transmitting medium to provide a pressure signal representing variations in the physiologic pressure on the electrical wires; signal processing and telemetry circuitry coupled to the electrical wires to receive the pressure signal and provide a telemetry signal representing the pressure signal; and a housing holding the signal processing and telemetry circuitry, wherein the transducer is remote from the housing.
 2. The apparatus of claim 1 wherein the pressure transmission catheter has a length short enough to avoid significant head pressure artifact and provide sufficient dynamic response.
 3. The apparatus of claim 2 where the length of the pressure transmission catheter is long enough to accommodate surgical limitations and tolerance concerns.
 4. The apparatus of claim 1 wherein the pressure transmission catheter has a length in the range from approximately two millimeters to approximately four centimeters.
 5. The apparatus of claim 1 wherein the pressure transmitting medium comprises a gel.
 6. The apparatus of claim 1 wherein the pressure transmitting medium comprises a gel and a low-viscosity liquid.
 7. The apparatus of claim 1 wherein the pressure transmitting medium comprises a slidable plug and a low-viscosity liquid.
 8. The apparatus of claim 1 wherein the transducer is pre-temperature compensated and disposable.
 9. The apparatus of claim 1 further comprising a sensor for measuring a temperature of the transducer to be provided to a computing device for compensating the temperature.
 10. The apparatus of claim 1 wherein the housing is implantable remote from the area having the physiological pressure.
 11. The apparatus of claim 1 wherein the signal processing and telemetry circuitry transmits the telemetry signal to an external receiver.
 12. The apparatus of claim 1 wherein the apparatus can be employed to measure venous pressure, pulmonary pressure, bladder pressure, or intracranial pressure without significant head pressure artifact and with a sufficient dynamic response.
 13. The apparatus of claim 1 wherein the pressure transmission catheter includes: a lumen filled with the pressure transmitting medium; a inner layer material surrounding the lumen; and an outer layer material surrounding the inner layer material, wherein the outer layer material has a different hardness than the inner layer material.
 14. The apparatus of claim 13 wherein the inner layer material is harder then the outer layer material.
 15. An apparatus for measuring physiological pressure comprising: a pressure transmission catheter filled with a pressure transmitting medium and having a distal tip for placement in an area having a physiological pressure and a proximal end; a connecting catheter carrying electrical wires; a transducer coupled to the electrical wires and in communication with the pressure transmitting medium at the proximal end of the pressure transmission catheter to provide a pressure signal representing variations in the physiologic pressure on the electrical wires; signal processing circuitry coupled to the electrical wires to process the pressure signal; and wherein a distance between the distal tip and the proximal end of the pressure transmission catheter is sufficiently short to avoid significant head pressure artifact and provide sufficient dynamic response yet sufficiently long to accommodate surgical limitations and tolerance concerns in applications for measuring physiological pressures where, if a length of the pressure transmission catheter was the combined length of the pressure transmission catheter and the connecting catheter, there would be a significant head pressure artifact and/or an insufficient dynamic response.
 16. The apparatus of claim 15 further comprising: a housing holding the signal processing circuitry, wherein the transducer is remote from the housing.
 17. The apparatus of claim 16 further comprising: telemetry circuitry in the housing coupled to the signal processing circuitry to provide a telemetry signal representing the pressure signal.
 18. The apparatus of claim 15 wherein the distance between the distal tip and the proximal end of the pressure transmission catheter is in the range from approximately two millimeters to approximately four centimeters.
 19. The apparatus of claim 15 wherein the pressure transmitting medium comprises a gel.
 20. The apparatus of claim 15 wherein the pressure transmitting medium comprises a gel disposed at the distal tip and a low-viscosity liquid from the gel to the proximal end.
 21. The apparatus of claim 15 wherein the pressure transmitting medium comprises a slidable plug disposed at the distal tip and a low-viscosity liquid from the slidable plug to the proximal end.
 22. The apparatus of claim 15 wherein the transducer is pre-compensated and disposable.
 23. The apparatus of claim 15 wherein the housing is implantable remote from the area having the physiological pressure.
 24. The apparatus of claim 15 wherein the signal processing and telemetry circuitry transmits the telemetry signal to an external receiver.
 25. The apparatus of claim 15 wherein the apparatus can be employed to measure venous pressure, pulmonary pressure, bladder pressure, or intracranial pressure without significant head pressure artifact and with a sufficient dynamic response.
 26. The apparatus of claim 15 wherein the pressure transmission catheter includes: a lumen filled with the pressure transmitting medium; a inner layer material surrounding the lumen; and an outer layer material surrounding the inner layer material, wherein the outer layer material has a different hardness than the inner layer material.
 27. The apparatus of claim 26 wherein the inner layer material is harder then the outer layer material.
 28. An apparatus for measuring physiological pressure comprising: a pressure transmission catheter filled with a pressure transmitting medium and having a distal tip for placement in an area having a physiological pressure and a proximal end, wherein a distance between the distal tip and the proximal end is sufficiently short to avoid significant head pressure artifact and provide sufficient dynamic response yet sufficiently long to accommodate surgical limitations and tolerance concerns; a connecting catheter carrying electrical wires; a transducer coupled to the electrical wires and in communication with the pressure transmitting medium at the proximal end of the pressure transmission catheter to provide a pressure signal representing variations in the physiologic pressure on the electrical wires; signal processing circuitry coupled to the electrical wires to process the pressure signal; and a housing holding the signal processing circuitry, wherein the transducer is remote from the housing.
 29. The apparatus of claim 28 further comprising: telemetry circuitry in the housing coupled to the signal processing circuitry to provide a telemetry signal representing the pressure signal.
 30. The apparatus of claim 28 wherein the distance between the distal tip and the proximal end of the pressure transmission catheter is in the range from approximately two millimeters to approximately four centimeters.
 31. The apparatus of claim 28 wherein the pressure transmitting medium comprises a gel.
 32. The apparatus of claim 28 wherein the pressure transmitting medium comprises a gel disposed at the distal tip and a low-viscosity liquid from the gel to the proximal end.
 33. The apparatus of claim 28 wherein the pressure transmitting medium comprises a slidable plug disposed at the distal tip and a low-viscosity liquid from the slidable plug to the proximal end.
 34. The apparatus of claim 28 wherein the transducer is pre-compensated and disposable.
 35. The apparatus of claim 28 wherein the housing is implantable remote from the area having the physiological pressure.
 36. The apparatus of claim 29 wherein the signal processing and telemetry circuitry transmits the telemetry signal to an external receiver.
 37. The apparatus of claim 28 wherein the apparatus can be employed to measure venous pressure, pulmonary pressure, bladder pressure, or intracranial pressure without significant head pressure artifact and with a sufficient dynamic response.
 38. The apparatus of claim 28 wherein the pressure transmission catheter includes: a lumen filled with the pressure transmitting medium; a inner layer material surrounding the lumen; and an outer layer material surrounding the inner layer material, wherein the outer layer material has a different hardness than the inner layer material.
 39. The apparatus of claim 38 wherein the inner layer material is harder then the outer layer material.
 40. A pressure transmitting catheter implantable in an area having a physiological pressure, the pressure transmitting catheter comprising: a lumen filled with a pressure transmitting medium; a inner layer material surrounding the lumen; and an outer layer material surrounding the inner layer material, wherein the outer layer material has a different hardness than the inner layer material.
 41. The apparatus of claim 40 wherein the inner layer material is harder then the outer layer material.
 42. The pressure transmitting catheter of claim 40 wherein a transition between the inner layer material and the outer layer material is a sharp transition.
 43. The pressure transmitting catheter of claim 40 wherein a transition between the inner layer material and the outer layer material is a gradient transition.
 44. The pressure transmitting catheter of claim 40 wherein the harder one of the inner and outer layer materials essentially determines the frequency response of the pressure transmitting catheter and the softer one of the inner and outer layer materials makes the pressure transmitting catheter more flexible and kink resistant.
 45. The pressure transmitting catheter of claim 40 wherein the inner layer material comprises 72 D urethane and the outer layer material comprises 80 A urethane.
 46. The pressure transmitting catheter of claim 40 wherein an outer diameter of the pressure transmitting catheter is in a range from approximately 0.014 to approximately 0.022 inches, the inner diameter of the lumen in less than approximately 0.008 inches, and the combined thickness of the inner material and outer material is less than approximately 0.005 inches.
 47. The pressure transmitting catheter of claim 46 wherein the thickness of the inner material is less than approximately 0.002 inches.
 48. The pressure transmitting catheter of claim 40 wherein the inner layer material comprises a radiopaque material.
 49. The pressure transmitting catheter of claim 48 wherein the inner layer material comprises an approximately 20-30% barium filled urethane.
 50. The pressure transmitting catheter of claim 48 wherein the outer layer material comprises non-thrombogenic material.
 51. An apparatus for measuring physiological pressure comprising: a pressure transmission catheter implantable in an area having a physiological pressure and including: a lumen filled with a pressure transmitting medium, a inner layer material surrounding the lumen, and an outer layer material surrounding the inner layer material, wherein the outer layer material has a different hardness than the inner layer material; a connecting catheter carrying electrical wires; a transducer coupled to the electrical wires and in communication with the pressure transmitting medium to provide a pressure signal representing variations in the physiologic pressure on the electrical wires; signal processing circuitry coupled to the electrical wires to process the pressure signal; and a housing holding the signal processing circuitry, wherein the transducer is remote from the housing.
 52. The apparatus of claim 51 wherein the inner layer material is harder then the outer layer material.
 53. The apparatus of claim 51 wherein the pressure transmission catheter has a distal tip for placement in the area having a physiological pressure and a proximal end coupled to the transducer, and wherein a distance between the distal tip and the proximal end is sufficiently short to avoid significant head pressure artifact and provide sufficient dynamic response yet sufficiently long to accommodate surgical limitations and tolerance concerns.
 54. An apparatus for measuring physiological pressure comprising: a pressure transmission catheter filled entirely with a pressure transmitting gel and implantable in an area having a physiological pressure; a connecting catheter carrying electrical wires; a transducer coupled to the electrical wires and in communication with the pressure transmitting gel to provide a pressure signal representing variations in the physiologic pressure on the electrical wires; signal processing and telemetry circuitry coupled to the electrical wires to receive the pressure signal and provide a telemetry signal representing the pressure signal; and a housing holding the signal processing and telemetry circuitry and the transducer.
 55. The apparatus of claim 54 wherein the pressure transmission catheter includes: a lumen filled with the pressure transmitting gel; a inner layer material surrounding the lumen; and an outer layer material surrounding the inner layer material, wherein the outer layer material has a different hardness than the inner layer material.
 56. The apparatus of claim 55 wherein the inner layer material is harder then the outer layer material.
 57. An apparatus for measuring physiological pressure comprising: a pressure transmitting catheter having a lumen filled with a pressure transmitting medium; a transducer in communication with the pressure transmitting medium to provide a pressure signal representing variations in the physiological pressure on an electrical wire; signal processing circuitry coupled to the electrical wire to process the pressure signal; and a housing holding the signal processing circuitry and the transducer; wherein the pressure transmitting catheter includes a first layer material surrounding the lumen and at least one additional layer of material surrounding the first layer material, wherein the at least one additional layer of material has at least one material with a different hardness than the first layer of material.
 58. The apparatus of claim 57 wherein the pressure transmitting medium comprises a gel.
 59. The apparatus of claim 57 wherein the pressure transmitting medium comprises a gel and a low-viscosity liquid.
 60. The apparatus of claim 57, wherein the lumen is filled entirely with a pressure transmitting gel.
 61. The apparatus of claim 57 further comprising telemetry circuitry located in the housing and coupled to the signal processing circuitry to provide a telemetry signal representing the pressure signal.
 62. The apparatus of claim 61 wherein the telemetry circuitry transmits the telemetry signal to an external receiver.
 63. The apparatus of claim 57 wherein the apparatus can be employed to measure arterial pressure, venous pressure, pulmonary pressure, bladder pressure, left ventricle pressure, or intracranial pressure.
 64. An apparatus for measuring physiological pressure comprising: a pressure transmission catheter having a lumen filled entirely with a pressure transmitting gel and implantable in an area having a physiological pressure; a transducer in communication with the pressure transmitting gel and coupled to an electrical wire to provide a pressure signal representing variations in the physiologic pressure on the electrical wire; signal processing circuitry coupled to the electrical wire to receive the pressure signal and provide a signal representing the pressure signal; and a housing for holding the signal processing circuitry and transducer.
 65. The apparatus of claim 64, wherein the pressure transmitting catheter includes a first layer material surrounding the lumen and at least one additional layer of material surrounding the first layer material, wherein the at least one additional layer of material has at least one material with a different hardness than the first layer of material.
 66. The apparatus of claim 64, wherein the pressure transmitting catheter is flexible.
 67. The apparatus of claim 64 further comprising telemetry circuitry located in the housing and coupled to the signal processing circuitry to provide a telemetry signal representing the pressure signal.
 68. The apparatus of claim 67 wherein the telemetry circuitry transmits the telemetry signal to an external receiver.
 69. An apparatus for measuring physiological pressure comprising: a pressure transmission catheter having a lumen filled with a pressure transmitting medium and implantable in an area having a physiological pressure; a transducer in communication with the pressure transmitting medium and coupled to an electrical wire for providing a signal representing variations in the physiologic pressure on the electrical wire; and a connecting catheter carrying the electrical wire to a location remote from the transducer.
 70. The apparatus of claim 69 wherein the pressure transmission catheter has a length short enough to avoid significant head pressure artifact and provide sufficient dynamic response.
 71. The apparatus of claim 69 wherein the length of the pressure transmission catheter is long enough to accommodate surgical limitations and tolerance concerns.
 72. The apparatus of claim 69 wherein the pressure transmission catheter has a length in the range from approximately two millimeters to approximately four centimeters.
 73. The apparatus of claim 69 wherein the pressure transmitting medium comprises a gel.
 74. The apparatus of claim 69 wherein the pressure transmitting medium comprises a gel and a low-viscosity liquid.
 75. The apparatus of claim 69 wherein the pressure transmitting medium comprises a slidable plug and a low-viscosity liquid.
 76. The apparatus of claim 69 wherein the pressure transmitting medium comprises only a gel which fills the entire lumen.
 77. The apparatus of claim 69 wherein the pressure transmitting catheter includes a first layer material surrounding the lumen and at least one additional layer of material surrounding the first layer material, wherein the at least one additional layer of material has at least one material with a different hardness than the first layer of material.
 78. The apparatus of claim 69 further comprising signal processing and telemetry circuitry coupled to the electrical wire to receive the pressure signal and provide a telemetry signal representing the pressure signal.
 79. The apparatus of claim 78, wherein the signal processing and telemetry circuitry transmits the telemetry signal to an external receiver.
 80. The apparatus of claim 78, wherein the signal processing and telemetry circuitry is located within a housing and wherein the housing is remote from the transducer.
 81. An apparatus for measuring physiological pressure comprising: a pressure transmission catheter having a lumen filled entirely with a pressure transmitting gel and implantable in an area having a physiological pressure; and a transducer in communication with the pressure transmitting gel and coupled to an electrical wire to provide a signal on the electrical wire which represents variations in the physiologic pressure.
 82. The apparatus of claim 81, wherein the pressure transmitting catheter includes a first layer material surrounding the lumen and at least one additional layer of material surrounding the first layer material, wherein the at least one additional layer of material has at least one material with a different hardness than the first layer of material.
 83. The apparatus of claim 81, further comprising signal processing and telemetry circuitry coupled to the electrical wire to receive the pressure signal and provide a telemetry signal representing the pressure signal.
 84. The apparatus of claim 83, further comprising a housing holding the signal processing and telemetry circuitry and transducer.
 85. The apparatus of claim 81, wherein the electrical wire is carried within a connecting catheter carrying the electrical wire to a location remote from the transducer.
 86. The apparatus of claim 85, wherein the pressure transmission catheter has a length short enough to avoid significant head pressure artifact and provide sufficient dynamic response.
 87. The apparatus of claim 85, wherein the pressure transmission catheter has a length in the range from approximately two millimeters to approximately four centimeters.
 88. The apparatus of claim 85, wherein the length of the pressure transmission catheter is long enough to accommodate surgical limitations and tolerance concerns.
 89. The apparatus of claim 85, further comprising signal processing and telemetry circuitry coupled to the electrical wire to receive the pressure signal and provide a telemetry signal representing the pressure signal.
 90. The apparatus of claim 89, wherein the telemetry circuitry transmits the telemetry signal to an external receiver. 